Post-COP26, How Green Can We Make Concrete?

With the conclusion of COP26, many experts say the little headway made by the Glasgow Climate Pact will be insufficient to meet necessary global climate targets. The new deal reaffirms the Paris agreement to limit global temperature rise to 1.5 degrees C above pre–industrial temperatures and aims to reduce global carbon dioxide emissions “by 45% by 2030 relative to the 2010 level and to net zero around mid–century.” Unfortunately, one analysis from Climate Action Tracker reveals that, even if 2030 targets are met, the world will witness an elevation of 2.4 degrees C above pre–industrial levels by 2100. More urgent action is needed.

One specific sign of progress concerns construction materials. Recognizing that cement and steel together comprise about 15% of global CO₂ emissions, five nations participating in the COP26 summit agreed to reduce the combined footprint of these materials dramatically in the coming decades. Canada, Germany, India, the United Arab Emirates, and the U.K. pledged to attain net zero in the “major public construction” use of concrete and steel by 2050, meeting interim targets by 2030. Additionally, the Global Cement and Concrete Association reports that, in October, 40 global concrete and cement manufacturers committed to reduce CO₂ emissions by 2030 on the path to achieving net zero in 2050.

Concrete is the primary focus for reductions, given that it has the most significant CO₂ impact of any material (cement production is responsible for 8% of worldwide emissions). The resurgence of interest in timber is primarily due to its superior environmental performance to concrete: Trees absorb 1.65 to 1.80 kilogram of CO₂ per kilogram of material, and wood construction generally stores more carbon than is emitted. In contrast, concrete emits about 1 kilogram of CO₂ per kilogram of material, or much more carbon than it sequesters. Researchers are thus conducting blue-sky studies proposing the use of wood as a global carbon sink.

However, diminishing the global juggernaut that is the concrete industry will be no small feat given its pervasive and entrenched character. Timber is also not without challenges—as COP26’s halt to worldwide deforestation indicates. Furthermore, many concrete and cement manufacturers have been developing ways to reduce—and even store—CO₂ in new concrete. This trend raises the question: How good can concrete realistically get?

Since producing 1 ton of concrete generates close to 1 ton of CO₂, carbon sequestration offers the greatest promise for carbon reduction in concrete manufacture. The Sydney–based Calix flash-heats cement feedstock minerals in a reactor that allows for the separation and capture of the CO₂. This stored carbon dioxide is then available to be injected into concrete while it is being mixed, a process called reverse calcination that produces a stronger material than conventional concrete.

Other companies are employing CO₂ from other emitting industries to make concrete. For example, the University of California, Los Angeles-–based research spinoff CarbonBuilt has developed a method to inject the CO₂ captured from coal plant exhaust, and other industrial emissions, into a concrete mix to create concrete masonry units. Scottish firm Concrete Capture Machine has created an intermediate step, dissolving captured CO₂ into alkali to produce a carbonate solution that may be used to make concrete as well as plastics, paints, and other materials. The Canada–based CarbonCure is a leader in reverse calcination in concrete, injecting CO₂ from industrial gas companies in concrete mixes at more than 400 concrete plants. Los Gatos, Calif.–based Blue Planet Systems produces CO₂–storing aggregate for concrete production. One metric ton of aggregate sequesters 440 kilogram of CO₂—nearly half the amount produced to make one metric ton of concrete.

Environmental performance varies based on the volume of emissions that can be stored in the concrete, the processing required to capture the CO₂, the amount of concrete necessary, and the energy source used. Today, carbon capture, use, and storage (CCUS) approaches can lock away 5% of the CO₂ emissions generated to make concrete, but this amount could theoretically reach 30% with new technologies, according to McKinsey. Some carbon capture methods, such as Calix’s approach, deliver pure CO₂—whereas others necessitate filtration and cleanup. Today, many concrete manufacturers are experimenting with emission–reducing formulas, such as the substitution of fly ash for a portion of the Portland cement. According to CO₂ Concrete, the carbon intensity (CI) of its specially formulated concrete blocks is up to 65% less than standard concrete.

Other products, such as ultra–high–performance concrete like Ductal, minimize the material used for a given purpose. As for the energy source: cement made in electric kilns powered by solar or wind energy, industrial waste, or biomass will outperform that made with fossil fuels. Advocates of the latter case, called bioenergy with carbon capture and storage (BECCS), argue that materials like wood are one of the best fuel sources to make concrete since the CO₂ they release originated from the atmosphere.

In combination, these approaches could lead to a 75% reduction of CO₂ emissions for the cement industry by 2050, according to McKinsey. Notably, only about 20% of the reduction will be based on incremental advances such as energy–efficient kilns and Portland cement substitutes; the remainder will require innovative approaches such as the CO₂–capture methods described above. The approximately 33 billion tons of concrete produced annually today result in roughly the same quantity of CO₂ emissions. However, in McKinsey’s 2050 best–case scenario, the world could consume the same amount of concrete with only 8.25 billion tons of CO₂ emitted—the equivalent of 2% of the total global emissions today.

This achievement is not the net–zero objective planned by the Global Cement and Concrete Association, presumably because the think tank is making a more conservative—and perhaps realistic—assessment in its analysis. Still, the promising recent advances in the industry suggest that the more ambitious aspirations of the GCCA may yet be attainable.

This article has been updated since first publication.